1. Field of the Invention
This invention relates to local oscillators for radio receivers, and to methods of controlling and adjusting such local oscillators.
2. Description of the Prior Art
In certain types of radio receiver, it is necessary to provide a local oscillator with very high stability, namely an oscillator which will maintain its frequency characteristics without drift. One example of such a requirement arises in relation to radio-paging receivers. In such cases, a crystal oscillator has generally been utilized and this will typically provide stability down to about 5 ppm. However, even this order of stability is sometimes insufficient. Variations in the characteristic frequency of a crystal oscillator arise as a result of changes in the ambient temperature. Accordingly, it has previously been proposed to measure the performance of a crystal oscillator with variation in temperature, and then to construct a control unit to provide compensation for the oscillator so as to stabilize its frequency. In one proposal, a voltage-controlled oscillator is provided together with a temperature detector, and the control unit is adjusted so as to provide a control voltage to the oscillator depending on the detected temperature such that the oscillator frequency is maintained constant, the control voltage for each temperature having been derived by testing during manufacture. This technique can be slow and expensive. Moreover, since it relies on a single test procedure during manufacture when the crystal oscillator is new, it cannot take account of ageing effects as a result of which the oscillator performance will drift from its original frequency characteristics over time. Thus, the performance of a radio receiver incorporating a local oscillator of this type will deteriorate over time.